1. Field of the Invention
The present invention relates to an exhaust emission control device for decreasing nitrogen dioxides contained in the exhaust gas of an internal combustion engine by means of a reducing agent to purify the exhaust gas.
2. Description of Related Art
An exhaust emission control device has been used to purify the exhaust gas emitted from an internal combustion engine. With this device, nitrogen oxides (NOx) contained in the exhaust gas are deoxidized by a reducing agent. This reducing agent is, for example, formed of urea-containing water. When ammonia (NH3) produced by decomposition of urea (CO(NH2) 2) reacts with nitrogen oxides on catalysts, the nitrogen oxides are deoxidized to harmless nitrogen (N2) and water (H2O) Therefore, nitrogen oxides contained in the exhaust gas are considerably decreased, so that the exhaust gas is purified.
Japanese Patent Specification No. 3686666 discloses an exhaust emission control device. This device has an additive valve directly attached to an exhaust piper the reducing agent is directly injected from the additive valve into the exhaust pipe so as to be dispersed in the exhaust gas. Therefore, the reducing agent is mixed with the exhaust gas, and the exhaust gas is purified.
However, because the flow rate of the exhaust gas in the exhaust pipe constantly fluctuates in response to the operation conditions of the engine, it is difficult to efficiently purify the exhaust gas by means of the reducing agent. For example, when the flow rate of the exhaust gas is lowered, the reducing agent injected from the additive valve reaches specific portions of catalysts and/or a pipe wall without being sufficiently dispersed in the exhaust gas. Therefore, the reducing agent is not efficiently used to purify the exhaust gas.